The invention relates generally to an apparatus and method for the manufacture of multilayer articles. The invention is particularly concerned with the manufacture of multilayer tablets such as are used, for example, for medicinal purposes.
A known apparatus for the manufacture of multilayertablets has a rotary disc on which there are mounted several female dies. A minimum of two feeding devices for supplying the female dies with flowable material are arranged adjacent the rotary disc. Compressing devices are provided and cooperate with the female dies to compress the material fed into these dies. The compressing devices also cause adjacent layers to adhere to one another.
With apparatus of this type, control purposes require that it be possible to determine the weight of the first compressed layer. This is necessary since the weight of the second layer, which is pressed onto the first layer and is often constituted by the working substance of the tablets, is determined on the basis of the weight of the first layer.
The control tests run until now have been expensive and, in practice, have also sometimes been found to be inadequate. The reasons for this are as follows:
In principle, it would be possible to eject the compressed first layer of a tablet from its female die via an ejector during rotation of the rotary disc and to perform a measurement on this layer. The ejection could be accomplished after the layer leaves the first compressing station which follows the first feeding station in the direction of rotation of the rotary disc. In practice, however, this procedure does not work. The reason resides in that the first layer is not highly compressed by the first compressing station in order that it may be made to adhere to the second layer. Accordingly, the first layer disintegrates upon being ejected from the female die.
On the other hand, if the first layer were permitted to pass through a second feeding station and the associated second compressing station so that a higher degree of compression is obtained, then a second layer is formed before ejection from the female die. In such an event, a weight measurement would not yield information on the weight proportion of the first layer in relation to the second layer, that is, in relation to the weight of the working substance of the tablet.
One attempt to overcome the above problem has involved increasing the pressure exerted by the first compressing station during passage therethrough of a first layer intended for control purposes. This procedure, however, exhibits the disadvantage that it becomes necessary to readjust the compressing force downwardly after the test sample, which has been compressed to a greater degree than usual for a first layer, has been removed. The reason that such a readjustment is required resides in that a satisfactory bond between a first layer and a subsequently deposited second layer can be obtained only when the first layer has not been too highly compressed. The problem with readjusting the compressing force is that there is no assurance that the values obtained during the control tests apply to tablets which are manufactured under different pressure conditions.